Plants that have been difficult or impossible to root from cuttings are generally propagated by a number of methods that include air-layering or marcotts; layering in stool beds; girdling and/or banding several weeks before taking cuttings and stem etiolation. All of these techniques require hand labor on each individual plant being produced.
Stem etiolation involves forcing new shoot growth under conditions of heavy shade or total darkness and then using this growth as the cutting propagule. Reid, O. "The Propagation of Camphor By Stem Cuttings", Trans. Bot. Soc. Edin. 28:184-188 (1923). Gardener, F. E., "Etiolation As A Method of Rooting Apple Variety Stem Cuttings", Proc. Am. Sco. Hort. Sci. 34:323-329 (1936). The use of etiolation both alone and in conjunction with banding is discussed extensively in U.S. Pat. No. 4,885,868.
In trench layering, vegetatively propagated dormant plants of the desired variety are either planted out-of-doors horizontally under the soil, or planted vertically out-of-doors. If planted vertically, its branches are bent to a horizontal position and covered with soil. In both of these situations, the buds begin to grow in the spring, and the shoots grow up through the soil. These shoots have tops which grow in the air and light providing the rest of the plant with carbohydrates; and have bases which become etiolated and are therefore more inclined to root. The shoots are left in place to root, and are not harvested until the fall or the spring of the following year. Although they may also be treated with a rooting hormone, they are not cut and placed in a mist bench. Their rooting rates are therefore not as high as experienced in the method of this invention, and since shoots are not removed as they are produced, some of the potentially harvestable shoots never form because the initiation of bud growth is inhibited by the growing shoots. Therefore, the number of harvested shoots is also lower than in the claimed method. Another difference between the two methods is that trench layering is performed out-of-doors, while the claimed method is performed in a greenhouse. The conditions during the plants' growth can therefore be much better controlled in the method of this invention.
Air layering is performed on a stock plant of the desired variety, on a branch in the air, thus resulting in its name. A portion of the branch is "girdled" completely around its circumference at a point just below the section to be layered. Then the section of branch immediately above the girdle (away from the trunk) is usually treated with a rooting hormone. This section is next covered with a material capable of holding water, such as sphagnum moss, and last is wrapped with a material which is impenetrable by both water and light such as tar-coated paper or aluminum foil. The girdle allows water to be transported upward from the roots to the part of the branch above the cut, but it stops the transport of carbohydrates and rooting hormones downward past the cut. These chemicals therefore accumulate just above the cut and promote rooting. The wrapping material holds in water to keep the branch and newly forming roots hydrated, but it also etiolates the branch, and thus increases the rooting potential of this plant part.
Descriptions of the trench and air layering techniques are known in the art of horticulture, Hartmann, H. G. et al, Plant Propagation: Principles and Practice, Prentice Hall: Fifth Edition (1990).
Banding, described in U.S. Pat. No. 4,885,868, is a technique similar to air layering. In this procedure, a stock plant is grown in the dark so that the emerging shoots are etiolated. A band of light-excluding material is wrapped around the bases of the growing shoots, and the plant is then placed in the light. The uncovered portions of the shoots then develop chlorophyll and thus become unetiolated, while the covered portions stay etiolated. Thus, carbohydrates are supplied for plant growth while producing shoots with etiolated bases which have the propensity to root well. Both procedures work well and produce cuttings which root at high rates. The significant difference between banding and the method of this invention is that in banding, a hand operation is required for each shoot to be etiolated, while in the method of this invention, the shoots are in effect self-etiolated, requiring no hand operations other than the original burying of the stock plant. And, as in trench layering, the whole operation must be repeated to produce new etiolated shoots in the banding technique.
U.S. Pat. No. 4,012,866 describes a technique which has been the standard of the industry for producing "clonal rootstocks" of avocados. It is performed by grafting a "scion" of the desired variety, the clonal rootstock, on top of a mother seedling. This grafted plant is fitted with a metal collar above the graft, and planted in the ground, with the ground level above the metal ring which slowly girdles the plant as it grows and expands. As in the case of air layering, the accumulation of carbohydrates and rooting hormones above the graft eventually induces the scion to produce roots. The mother plant either dies or is later cut off and discarded. The scion which is now rooted then becomes a clonal rootstock on top of which is grafted a scion of the variety desired to produce avocado fruit. This relates to the method of the present invention mainly in that it is another procedure which utilizes light exclusion to enhance rooting. But whereas the technique described in U.S. Pat. No. 4,012,866 produces only one clonal rooted plant from several hand labor operations, the method of this invention produces mainly rootable cuttings from the one act of burying the stock plant horizontally.
A method of tissue culturing of nut species (most of which are considered hard-to-root) that has produced viable plants is a technique where the culture vessel is painted black, to exclude light from its base and the culture medium is described by Rugina, E. A. et al, Scientis Horticulturae 53:63-72 (1993). Also, the surface of the culture medium is covered with black, light-blocking beads. These beads are sterilized before introduction into the interior of the vessel. The bases of the plants in the vessel are etiolated as they grow, and therefore are primed to be more rootable either in a root-promoting culture medium, or after being removed and treated with a rooting hormone, and being placed in a mist bench or fogged greenhouse.
Molnar et al, U.S. Pat. No. 4,569,914, discloses a method for the production of propagating material of plants by micropropagation which require that they begin with sterile shoot cuttings and not stock plants.
Hechinger, U.S. Pat. No. 3,012,372, discloses a method for the vegetative propagation of plants that is basically a modification of trench layering or stool bedding. Hechinger discloses only fruit trees as being rooted. Apples, pears, plums, cherries, and peaches, which are the species listed, have all been rooted by traditional methods, such as commercial stool beds, and so are not considered impossible or hard-to-root from cuttings such as all of the nut species.
Raines, U.S. Pat. No. 2,431,890, describes a humidifying/misting chamber of two sections where rooting and non-rooting portions can be treated differently. It mentions wrapping the bottom rooting portion in sphagnum moss, and providing stronger lighting for the top portion. But all of these shoots are treated vertically, and the operations are all done after the shoot (cutting) is taken.
Driver, U.S. Pat. No. 4,612,725, discloses a method for producing cuttings which root in place in the field. It mainly involves special enclosures with irrigation water piped to each enclosure. Also described is the work done in the tissue culture lab and the acclimation necessary in the greenhouse. This is a labor-intensive and time-consuming procedure which, even after all the work, does not produce a very good root system.
The Mlieva Orchards reference (S.U. Patent No. 1255-080-A) discloses a procedure for propagating apples, as pointed out above, a relatively easy to root species. In this process, the stock plant is placed in the soil in a curved position. One end of the stock plant is above the ground and the other end is inclined in the soil. This increases the amount of labor required to practice the method.
Bordovsky, Czech. Patent No. 76 678, discloses a method for the vegetative propagation of plants wherein a clonal stock plant is positioned in ground, etiolated shoots grown on the stock plant and rooted shoots are removed and planted. However, this method requires that the stock plant be placed in the ground at a slightly upward angle which increases the amount of labor required.
Mehra-Palta, U.S. Pat. No. 4,550,528, discloses a method wherein stock plants are used to produce shoots and the shoots removed and rooted in a mist bench. This procedure is referred to as hedging. In this method, stock plants are grown vertically rather than horizontally.
Raines, U.S. Pat. No. 2,431,890, discloses the use of a high moisture environment in the rooting of cuttings and Shimazu, U.S. Pat. No. 3,835,584, discloses the rooting of cuttings in a synthetic medium.
Pruitt, U.S. Pat. No. 2,988,441, Porte, U.S. Pat. No. 3,831,317, Schmitz et al, U.S. Pat. No. 2,816,825 and Nelson, U.S. Pat. No. 2,891,355, all disclose the use of synthetic root mediums for plant culture.
None of the prior art noted above or any other known to the inventor discloses or suggests the method of this invention.